## Kohn-Sham Equations

Using what we have just learned about taking derivatives of real functions of complex variables and including the normality constraint of each orbital with a separate Lagrange multiplier , the condition for the constrained minimization of (12) is We now take this derivative term by term.

The kinetic energy (10) has only one term in it, so the derivative is just what this term multiplies, For the electron-nuclear potential energy (3), the only term which depends on is the charge density, where multiplies . The nuclear potential is unchanged as varies, so the final term is just The electron-electron energy has a very similar structure. The only difference is that, here, when we change , the potential function also changes because it depends on . The net effect of the change in is the same as that of the direct change in . To see this we note that Poisson's equation implies also that . Thus, where we have moved the from acting on to acting on by integrating by parts twice. Since both terms are equal, we can take just twice the term, For the exchange-correlation term, we have already derived in (14) that . By the chain rule we just need to multiply this by for the result Fortunately, depends only on the nuclear positions and does not change with , so And, finally, only appears once in the constraint term, making the derivative, Summing all of these contributions, setting the resulting equation to zero, moving the '' term to the right-hand side, and dividing through by , we get the final result, Fortunately for us, this is in the form of a very well-known equation for which there are standard techniques. This is in the form of the standard Schrödinger equation, (16)

where we define the potential term as (17)

and we define . We interpret the potential as just the sum of the nuclear potential, the electrostatic potential created by the electrons, and an extra, exchange-correlation'' potential correction, . Since the Lagrange-multipliers are unknown constants at the start, we may as well think in terms of the instead, which have the interpretation of the Schrödinger energies for each orbital.

Tomas Arias 2004-01-26